הלכה אויף תהילים 137:5

And they also ordained that one who sets a table to make a meal for guests lack a little from it and leave an empty place without one of the bowls that it is fit to place there; and that when a women makes [herself up with] silver and gold jewelry, she leaves off one of the types of jewelry to which she is accustomed, such that the jewelry not be complete; and that when a groom marries a woman, he takes burnt ashes and puts it on his head in the place of where the tefillin lay. And all of these things are so as to remember Jerusalem, as it is stated (Psalms 137:5-6), "If I forget you, O Jerusalem, let my right hand wither. Let my tongue stick to my palate if I cease to think of you, if I do not raise up Jerusalem above my highest joy."

After the Beis Hamikdash was destroyed, our Sages, of blessed memory, ordained that on every joyous occasion, there should be a remembrance of the destruction of the Beis Hamikdash, as it is written: "If I forget you, Jerusalem, if I fail to elevate Jerusalem above my foremost joy" (Psalms 137:5). They decreed that no Jew should build a house¹It is, however, permitted to build a Beis Midrash or Synagogue without any restrictions. (Biyur Halachah 560:1) for himself that is lime-plastered and decorated like a royal palace.²Tur rules that if a house was merely painted, but not plastered, even though it was decorated, leaving an amah unpainted is sufficient. (see Mishnah Berurah 560:1) Neither should he lime plaster his entire house, but he may plaster the house,³If, however, the house was plastered with mud or clay, but not painted, it is not required to leave an amah unplastered. (Ibid. 560:2) with clay and then lime plaster, it (lit. whitewash it), and he should leave a space of one square amah⁴See glossary. opposite the entrance⁵Others have the custom to leave the amah above the door, because the main objective is that the inhabitants of the house see it and be reminded of the destruction, rather than those (strangers who enter the house on occasion). Leaving it opposite the entrance, only serves as a reminder, upon entering the house, but not as a reminder for those who spend time in the house. (Ibid. 560:3) unplastered,⁶It is not proper to have it painted black. Those who have curtains (wall-paper) around all the walls of the house, should also leave an amah of bare walls. (Ibid. 560:3) as a reminder of the destruction [of the Beis Hamikdash. ] We do not know of any good reason why people do not observe this custom nowadays.

And similarly, they decreed that one who lays a table for guests should leave something missing and leave a place without a bowl (that would have been suitable to place there). And when someone makes ornaments/jewellery of silver or gold, they should leave one missing from the kinds of ornament which are customary, so the ornament should not be complete. And when the groom marries a woman, take ashes from burnt food and place them on his head, in the place where the Tefilin rest. Gloss: There are places where they are accustomed to breaking a cup during the wedding ceremony, or placing a black cloth or other such items of mourning on the head of the groom. And all these things are in order to remember Jerusalem, as it is said, "If I forget you, Jerusalem...if I do not raise up Jerusalem over the height [head] of my joyfulness..." [Psalm 137:5-6]